Portable, sensitive radio receivers of the type frequently employed with "manpack" equipment for military purposes require highly efficient DC to AC to DC power converters. Usually, the efficiency is attained by employing switching converters to supply required DC voltages and currents for receiver components, such as RF, IF, and audio frequency amplifiers, as well as a frequency synthesizer. Because the different receiver components have different power supply requirements, it is necessary to employ several switching converters to derive the required power supply voltages and currents in the most efficient manner.
Switching converters, however, inject noise into the component being powered. The noise occurs at the switching frequency fundamental and harmonics of the switching converter. Sometimes, harmonics as high as the 500th harmonic of the switching frequency fundamental can be injected by the switching converter into the receiver components. If the power supply is switched at a frequency of 15 kHz, the 500th harmonic has a frequency of 7.5 mHz; hence, it is apparent that there is a possibility of interference by the switching harmonics with many standard IF frequencies and lower frequency RF signal inputs.
In addition to the harmonics, there is a problem with intermodulation products when more than one switching converter is employed. This is because the plural switching converters generally operate independently of each other and inject different harmonic waves into the various components driven by them. The harmonic components injected by the different switched power supplies into different non-linear components (e.g., the receiver amplifiers and mixer) are mixed together to form intermodulation components.
It is, accordingly, an object of the present invention to provide a new and improved switched power supply system for a portable, battery operated radio receiver.
Another object of the invention is to reduce the distortion introduced by a plurality of switched power converters for various components of a portable battery powered radio receiver.
Another object of the invention is to provide a new and improved power supply system for a portable, battery operated radio receiver wherein intermodulation due to interference from a plurality of switched converters is minimized.